At present, probably the best anode for oxygen-evolution is that described in U.K. patent specification No. 1.399.576, having a coating containing a mixed crystal of tantalum oxide and iridium oxide. However, known electrodes of this type contain at least about 7.5 g/m.sup.2 of iridium so that despite their excellent performance in terms of over-voltage and lifetime, the high cost of iridium makes these electrode less attractive and in order to be competitive with cheaper anodes they must be operated at a relatively high anodic current density which necessitates various expedients in the cell design. Consequently, anodes made of solid lead, lead alloys, cobalt-silicon alloys and so forth are still used in many electrowinning plants despite the known disadvantages of such materials.
Massive anodes of lead and lead alloys such as lead-silver thus remain the most widely used anodes in metal electrowinning from sulphate electrolytes despite problems of weight, poor mechanical strength and high over-voltage. Lead-silver alloys offer a lower over-voltage and are less subject to corrosion in zinc electrowinning, but massive anodes of these alloys are very expensive.
French Pat. No. 2,399,490 has proposed an electrowinning anode consisting of tubes of aluminum coated with an extrusion drawn lead-silver alloy; this seeks to provide a lightweight and relatively inexpensive structure compared to conventional massive lead-silver anodes, but is subject to numerous drawbacks. Firstly, the extrusion drawing process is expensive, does not provide an excellent electrical contact and is limited to tubular and rod-like sections. Secondly, in electrowinning applications the rod-like configuration results in a voltage penalty of about 350 mV compared to massive anodes, as well as a greater dissolution of lead leading to increased contamination of the cathodically deposited metal, e.g. zinc.